1. Technical Field
The present invention relates to a central office line interfacing circuit of an exchange, and more particularly to a circuit for detecting a hook-off and a ring signal.
2. Related Art
In general, an exchange such as a private branch exchange (PBX) or a key telephone system includes a ring detector for detecting a ring signal and a hook-on/off sensing circuit for sensing a hook-on/off status. A controller of the exchange checks a status of the signals generated from the ring detector and the hook-on/off sensing circuit to perform the incoming and outgoing call services.
In such a system of the prior art, if a telephone line connected between a central office exchange and a private branch exchange is in a normal state, turning on of a hook switch in an outgoing call mode results in formation of a central office line loop via the loop current from the central office exchange. However, when the telephone line is in an abnormal state, the central office line loop cannot be formed since the loop current from the central office exchange is cut off. This problem with prior art systems result from separate design of the ring detector and the hook-on/off sensor, such separate design resulting from the fact that the ring signal is typically a high voltage signal.
Thus, there is a need in the prior art for development of a central office line interface circuit which commonly detects a status of a ring signal and a hook switch in an exchange, and for commonly detecting the status of a ring signal and a hook switch in a telephone.
The following patents are considered to be representative of the prior art relative to the present invention, and are burdened by the disadvantage discussed above: U.S. Pat. No. 5,659,602 to Gay, entitled Telephone Apparatus For Caller ID, U.S. Pat. No. 5,619,567 to Apfel, entitled Variable DC Feed Characteristic In A Subscriber Line Interface Circuit, U.S. Pat. No. 5,594,788 to Lin et al., entitled Telephone System And Interface Device, U.S. Pat. No. 5,442,694 to Chitrapu et al., entitled Ring Tone Detection For A Telephone System, U.S. Pat. No. 5,402,482 to Minohara et al., entitled Ring Trip Deciding Circuit, U.S. Pat. No. 5,347,575 to Naseer et al., entitled Circuit To Detect The Hook Status And Ringing At The CPE End Of A Telephone Network, U.S. Pat. No. 5,335,271 to Takato et al., entitled Ring Trip Detection Circuit, U.S. Pat. No. 4,998,271 to Tortola et al., entitled Telephone Accessory, U.S. Pat. No. 4,731,827 to Wood, entitled Interswitch Line circuit, and U.S. Pat. No. 4,304,970 to Fahey et al., entitled Telephone Status Monitor Apparatus.
It is therefore an object of the present invention to provide a central office line interface circuit for commonly detecting a status of a ring signal and a hook switch in an exchange.
It is another object of the present invention to provide a central office line interface circuit for commonly detecting a status of a ring signal and a hook switch in a telephone.
According to an aspect of the present invention, a central office line interface circuit of an exchange includes a hook switch connected to a telephone line, a ring signal path connected in parallel to the hook switch, and a detection circuit connected between a controller of the exchange and a node of the hook switch and the ring signal path, so as to detect a ring signal in an incoming call mode and detect a hook off status in an outgoing call mode.